How to optimize resource usage when drawing image in a QWidget? - c++

Question

I'm subclassing the scrollAreaWidgetContents (which is literally a QWidget) of a QScrollArea and drawing an image on its background.
While resizing the widget, it was using too much CPU (tested with the QScrollArea containing no other widgets) so I tried to find a way to 'optimize' the drawing of the image, and I have written this class:
class ScrollChild : public QWidget
{
Q_OBJECT
public:
QTimer* m_resizeTimer;
int width;
int height;
QPixmap pixmap = QPixmap("...");
QPixmap pixmapScaled;
int pixWidth, prevpixWidth;
int pixHeight, prevpixHeight;
int pixmapPosX;
int pixmapPosY;
// getPos: a dynamic property set in Qt Designer to
// the widget that will be used as base to where the
// pixmap should be drawn.
QWidget* getPos;
QWidget* widget;
ScrollChild(QWidget* parent = 0) : QWidget(parent)
{
initTimer();
};
void resizeEvent(QResizeEvent* event)
{
QWidget::resizeEvent(event);
width = event->size().width();
height = event->size().height();
m_resizeTimer->start();
}
bool runtime = true;
void showEvent(QShowEvent* event)
{
if (runtime)
{
runtime = false;
// Looking for the property inside showEvent because
// when searched in the constructor its not found.
for (auto wdgt : this->findChildren<QWidget*>())
{
if (!wdgt->property("getPos").isNull())
{
getPos = wdgt;
break;
}
}
}
}
void initTimer()
{
m_resizeTimer = new QTimer(this);
m_resizeTimer->setInterval(100);
m_resizeTimer->setSingleShot(true);
connect(m_resizeTimer, &QTimer::timeout, this, &ScrollChild::resizePicture);
}
void resizePicture()
{
pixWidth = width;
pixHeight = height;
if (pixWidth > 600)
pixWidth = 600;
if (pixHeight > 400)
pixHeight = 400;
if ( (pixWidth != prevpixWidth) && (pixHeight != prevpixHeight) )
pixmapScaled = pixmap.scaled(pixWidth, pixHeight, Qt::KeepAspectRatioByExpanding, Qt::SmoothTransformation);
prevpixWidth = pixWidth;
prevpixHeight = pixHeight;
//qDebug() << "w: " << width << " h: " << height;
//qDebug() << "pixmap:\nw: " << pixmap.width() << "h: " << pixmap.height() << "\n";
repaint();
}
void paintEvent(QPaintEvent *e)
{
if (pixmapScaled.isNull())
return;
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
painter.setRenderHint(QPainter::SmoothPixmapTransform);
widget = getPos;
pixmapPosX = 0;
pixmapPosY = 0;
// Search the position of the widget marked in
// the Qt Designer.
// As the widget is inside a container we must
// iterate trought her parent adding the x pos
// relative to the scrollAreaWidgetContent.
while (widget != this)
{
pixmapPosX += widget->pos().x();
pixmapPosY += widget->pos().y();
widget = widget->parentWidget();
}
painter.drawPixmap(pixmapPosX, pixmapPosY,
pixmapScaled.width(), pixmapScaled.height(),
pixmapScaled);
}
};
It reduced the CPU usage by 60%~ while keeping the same looking/quality.
Its possible to improve on something else? or write it in a most efficient way?